The following protocol seeks to validate a non-invasive open circuit method for determination of cardiac output (pulmonary blood flow) at rest and during heavy exercise using simple gas exchange based measures. There are relatively few non-invasive techniques that give valid estimates of cardiac output at rest and during heavy whole body exercise. The acetylene rebreathing technique has been used extensively over the last 10-15 years and has been validated with thermodilution and Fick estimates of cardiac output in exercising humans and dogs. In this technique, the highly soluble gas, acetylene, is taken up in the lungs in direct proportion to the blood passing through the pulmonary circulation. A drawback to the rebreathing technique is the fact carbon dioxide builds up and oxygen decreases in both the rebreathing bag and lungs. The build up of CO2 causes increased dyspnea, particularly in patients with poor lung function and limited exercise tolerance. The transient changes in the VC02/V02 relationship make it difficult to control for changes in the lung-rebreathing bag volume which can lead to errors in the estimate of cardiac output. In addition, rebreathing may lead to alterations in breathing pattern, which also may cause changes in the cardiac output. Recently, an open circuit acetylene technique that offers several advantages over the rebreathing technique has been developed but not validated in humans. The present study proposes to validate the open circuit technique more fully in exercising humans against the already established technique, and the standard techniques to measure cardiac output using thermodilution and Fick. The specific aims of this study are to compare an open circuit acetylene uptake technique for determination of cardiac output with the traditional acetylene rebreathing technique; and to compare an open circuit acetylene technique for determination of cardiac output at rest and during exercise to the standard thermodilution and Fick measurements of cardiac output.